1. Field of the Invention
The invention relates to the production of stable, high-purity moldings from pyrogenic metal oxides without addition of binders and their use.
2. Description of Related Art
Pyrogenic metal oxides are extremely finely divided and have high specific surface areas, defined, spherical primary particles having a defined surface chemistry and no internal surfaces (pores). Furthermore, they have a very high chemical purity.
In view of the properties outlined above, pyrogenic silicon dioxides, for example, are attracting increasing interest as supports for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)).
However, owing to the particularly finely divided nature of the pyrogenic metal oxides, the production of moldings as are used, for example, as catalysts or catalyst supports from these pyrogenic metal oxides is difficult. The production of moldings from metal oxide powders is generally carried out by pressing or extrusion using binders and lubricants in order to obtain stable moldings. The binders and lubricants are inorganic and organic additives.
Inorganic additives such as magnesium stearate remain in the form of inorganic compounds, e.g. magnesium oxide, in the molding produced. Organic additives, too, can result in impurities such as carbon in the production process for the moldings. The desired very high purity of the pyrogenic metal oxides used, e.g. pyrogenic SiO2, is in this way lost in the moldings produced.
Apart from the high purity and the high surface area, it is further desired to obtain moldings having a very low bulk density. This can, firstly, have a favorable effect on mass transfer in the later catalyzed reaction, and secondly, enables a smaller mass of support material to be required in order to fill a particular reactor volume. The cost ratio of support material to reactor volume is improved in this way and the process becomes more economical.
Low bulk densities can be achieved, for example, by means of moldings which have at least one channel running through them, as in the case of, for example, rings. Ring-shaped bodies having a very low wall thickness are particularly useful. However, low wall thicknesses lead to moldings whose mechanical strengths are no longer sufficient for catalyst production and/or filling of the reactor and are therefore unsuitable as catalyst support materials.
Many possible ways of producing moldings from metal oxides are described in the prior art, but a binder is always added to achieve later strength.
EP 72390 describes the production of pressed bodies from a mixture of pyrogenic metal oxides, water, silica sol and a pressing auxiliary. A polyfunctional alcohol (e.g. glycerol) is claimed as an auxiliary.
EP 327722 discloses mixing pyrogenic silicon dioxide with kaolin and/or graphite, sugar, starch, urea, and aqueous wax in emulsion. The production of the pressed bodies can be carried out using punch presses, eccentric presses, extruders, rotary presses or compacters. An analogous procedure is used in EP 327815, but pyrogenic silicondioxide/aluminum oxide mixed oxide is used instead of pyrogenic silicon dioxide.
EP 393356 describes the production of pressed bodies from pyrogenic silicon dioxide, urea, methylcellulose and/or magnesium stearate, graphite, aluminum stearate and water.
EP 807615 describes a process for producing pressed bodies comprising pyrogenic silicon dioxide, methylcellulose, microwax and polyethylene glycol and water. The pressed bodies usually have contents of from 50 to 90% by weight of silicon dioxide, from 0.1 to 20% by weight of methylcellulose and from 0.1 to 15% by weight of microwax and from 0.1 to 15% by weight of polyethylene glycol.
DE-A-10247314 discloses moldings based on silicon dioxide and/or titanium dioxide which additionally contain glass fibers. The moldings are produced by homogenizing pulverulent silicon dioxide and/or titanium dioxide with glass fibers, methylhydroxypropylcellulose, wax emulsion or polyethylene glycol, polysaccharide and polyethylene oxide with addition of water. The resulting composition is shaped to produce extrudates.
According to DE4142898, it is possible to produce stable moldings from pyrogenic silica and aqueous-alcoholic ammonia solution. On the other hand, a purely aqueous ammonia solution does not lead to success. The high proportion of aqueous-alcoholic ammonia solution makes the mixture to be shaped strongly alkaline. The use of alcohol incurs the risk of C contamination of the resulting catalyst support. According to DE4142902, stable moldings can be obtained from pyrogenic silica and ammonia solution or from pyrogenic silica and alkali metal-containing silica sol only when the shaped bodies are subjected to hydrothermal treatment. In the case of the addition of ammonia, the mixture is again made very alkaline. It is known that this excess of base (pH >10) leads to partial dissolution of SiO2.
The documents of the prior art show that the production of stable moldings has hitherto not been possible without inorganic or organic additives such as extrusion auxiliaries, pore formers, sols or additional strengthening steps. All these measures cause a not inconsiderable proportion of undesirable contamination in the product.